Question

The purge stream may be used to replace a portion of the methane burned in the reformer. Assuming that the purge gas is at 35°C and that it is fed directly to the reformer burners with 5% excess air at 300°C, how much heat would be released per 100 kmol of purge gas? Estimate the savings ($ per kmol of purge gas) if the purge stream is used in place of natural gas for fuel. (Take $3 per million Btu as the approximate cost of natural gas.)

 

 

Answer 1

Given purge stream composition -

CH4- 6.4 %

CO2-6.3 %

CO- 6.8 %

H2- 78.7 %.

 

Please note that CO2 here will not combust further because carbon is alreaday in it\'s highest oxidation state possible here.

 

Following reaction will take place here-

CH4+ 2O2 - CO2+2H2O

CO+0.5O2 - CO2

H2+0.5O2 - H2O

 

for 100 moles of the purge gas.

Moles of CH4 = 6.4 moles

Moles of CO = 6.8 %

Moles of H2 = 78.7 %

 

We can take the data for the heat of combustion of these three reactions from the literature.

Heat of combustion of CO = - 283 kJ/mole

Heat of combustion of H2 = - 241.8 kJ/mole

Heat of combustion of CH4 = -217 kJ/mole

 

Please note that here negative sign is indicating that the heat is getting released.

Total heat released = 283*8.6 kJ +241.8*78.7 kJ + 217*6.4 kJ

                                   =22852 kJ

 

Amount of the purge gas that is being used = 46.8 kmol/h

                                                                              = 46800 moles/hr

 

For 1 hour

heat released by using 100 moles purge gas = 22852 kJ

Heat released by using this purge gas = (46800/100)*22852 kJ

                                                                    = 10694736 kJ

 

1 Btu =1.055 kJ

Heat released = (10694736 kJ )/(1.055 kJ/Btu)

                          = 10137190 Btu

                         = 10.13 Million Btu or approximately 10 Btu.

 

Saving if this purge stream is used in place of natural gas is 3$ per Btu.

Total savings = 3*10 $

                        = 30 $

 

Since we have done this calculation for 1 hour, savings will be reported as 30 $ per hour.

Since we have done this calculation for 1 hour, savings will be reported as 30 $ per hour.